1. Technical Field
The present invention relates to a technique of applying a voltage to drive a load.
2. Related Art
A technique of applying a voltage to drive a load of an electronic element such as a semiconductor element or a dielectric element has been widely used in various devices. In a fluid ejection device such as an inkjet printer for example, a voltage is applied to a piezo element that expands and contracts according to a voltage, so that fluid is pushed out of an ejection port and ejected. In a display device such as a liquid crystal display or an organic EL display, a voltage is applied to liquid crystal to align liquid crystal molecules, or a voltage is applied to an organic EL element to cause it to emit light, so that an image is displayed. Moreover, a technique of applying a voltage to drive various loads such as a motor or an electromagnet, in addition to the electronic element, has also been widely used.
In the devices that drive such various loads, the waveform of voltage (voltage waveform) to be applied to the load is controlled to control the operation of the load. Therefore, it is important to accurately generate a voltage waveform. For such a reason, a technique of generating a voltage waveform has been widely used in which a power supply that generates a voltage higher than the maximum voltage of a voltage waveform to be generated is prepared, and the voltage from the power supply is decreased using a semiconductor element such as a power transistor (for example, JP-A-11-259969). In this technique, the power transistor or the like can be controlled to accurately generate a voltage waveform.
Moreover, when a device includes a power supply having a high output voltage or a power transistor having a high withstand voltage, the device tends to be large. Therefore, a technique of using a circuit (so-called charge pump circuit) to omit a power transistor or the like for miniaturizing the device configuration has been proposed (JP-A-7-130484). The circuit charges a plurality of capacitors to the same voltage, connects the capacitors in series to increase the voltage, and changes the number of capacitors to be connected in series to change the voltage.
In the proposed technique, however, there is a problem that generating a highly accurate voltage waveform is difficult. That is, since the voltage is changed by changing the number of capacitors to be connected in series, the number of kinds of voltage values that can be output is limited to the number of capacitors. Since a voltage waveform has to be generated with the limited kinds of voltages, generating a highly accurate voltage waveform is difficult. The enhancement of accuracy of the voltage waveform is of course possible by providing more capacitors to output many kinds of voltages. This time, however, miniaturizing the circuit is difficult because the number of capacitors increases.